1. Field of the Invention
The invention concerns a method in order to generate an image data set by means of a magnetic resonance system. Moreover, the present invention concerns a correspondingly designed magnetic resonance system as well as a corresponding computer program product and an electronically readable data medium.
2. Description of the Prior Art
Magnetic resonance tomography lends itself to new fields of use by the acquisition of MR data with very short echo times TE (for example TE<500 μs). It is thereby possible to show substances or tissue that cannot be depicted by means of conventional sequences, for example a (T)SE (“(Turbo) Spin Echo”) sequence or a GRE (“Gradient Echo”) sequence, since their T2 time (the relaxation of the transverse magnetization of this substance or tissue) is markedly shorter than the echo time, and thus a corresponding signal from these substances or tissues has already decayed at the point in time of acquisition. With echo times that lie in the range of the corresponding decay time, it is possible for example to show bones, teeth or ice in an MR image although the T2 time of these objects lies in a range from 30-80 μs.
According to the prior art, sequences are known that enable a very short echo time. One example is the radial UTE (“Ultrashort Echo Time”) sequence as described, for example, in the article by Sonia Nielles-Vallespin “3D radial projection technique with ultrashort echo times for sodium MRI: Clinical applications in human brain and skeletal muscle”, Magn. Res. Med. 2007; 57; P. 74-81. In this sequence type the gradients are ramped up after a wait time T_delay after a non-selective or slice-selective excitation and the data acquisition is begun at the same time. The k-space trajectory scanned in such a manner after an excitation proceeds radially outwardly from the k-space center. Therefore, before the reconstruction (by means of Fourier reconstruction) of the image data from the raw data acquired in k-space these raw data must first be converted into a Cartesian k-space grid (for example by regridding).
An additional approach in order to enable short echo times is to scan k-space in points in that the free induction decay (FID) is detected. Such a method is also designated as a single point imaging since essentially only one raw data point in k-space is acquired for each RF excitation. One example of such a method for single point imaging is the RASP method (“Rapid Single Point (RASP) Imaging”, O. Heid, M. Deimling, SMR, 3rd Annual Meeting, Page 684, 1995). According to the RASP method, one raw data point in k-space, the phase of which was coded by gradients, is read out at a fixed point in time after the RF excitation at the “echo time” TE. The gradients are modified by means of the magnetic resonance system for each raw data point or, respectively, measurement point, and thus k-space is scanned point-by-point as is shown in FIGS. 1a and 1 b. 